CORC 1331 MAIN PAGE

PHYSICS TOPICS

Speed
Velocity
Acceleration
Constant Acceleration
Free Fall
Forces and Newton's Law
Kinetic Energy
Potential Energy

LIST OF EXAMPLES

CLASSIC PROBLEMS

Free Fall
Forces
Conservation of Energy
Conservation of Momentum

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Classic Problem in Free Fall

Here is an example of a typical question in the area of Free fall that students have difficulty solving:

Problem:
A ball is thrown upward with an initial velocity of 40 m/s.

Find:
a) the velocity of the ball at the highest point
b) the velocity of the ball after it has been in the air for 3 seconds
c) the change of the velocity after 1 second
d) the acceleration of the ball

Man throwing a ball

Solution:

a) Find the velocity of the ball at its highest point.

In order to solve the various parts of this problem you have to realize that when an object is moving upward or downward it's in vertical motion. In vertical motion there is a constant acceleration that is called acceleration due to gravity or gravitational acceleration which prevents any object from flying. Gravitational acceleration has a value of 9.8 m/s2 and is always directed downward. That's why our ball eventually is going to fall down.

In the beginning we threw the ball upward and therefore the initial velocity is also directed upward. Every second that the ball is in motion its velocity is going to decrease due to the different directions of the initial velocity (up) and gravitational acceleration (down) until it will reach the highest point where the velocity will be equal to zero. At this point the ball momentarily stops and then continues to move down. When this occurs the direction of the velocity of the ball and the gravitational acceleration are the same (down) and that's why the velocity will increase.

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 Question: What is the velocity of the ball at its highest point? Type your answer here: m/s

b) Find the velocity of the ball after it has been in the air for 3 seconds.

In order to calculate the change in the velocity we use the following formula:

Vf is the final velocity,
Vi is the initial velocity,
g is the gravitational acceleration and has a constant value of 9.8 m/s2,
t is the time interval.

In this part of the problem we have to calculate the final velocity of the ball after a three second time interval. So we will collect the data and plug the numbers into the formula.
Note :
We have to take into consideration the directions of the initial velocity (40 m/s upward) and the gravitational acceleration (9.8m/s2 downward). We usually choose upward direction as a positive and downward direction as a negative.

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 Question: What is the velocity of the ball after it has been in the air for 3 minutes? Type your answer here: m/s

c) Find the change of the velocity after 1 second.

When the ball is thrown upward its velocity is 40 m/s. Then due to gravitational acceleration, every second the velocity will decrease by 9.8 m/s until it reaches zero.

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 Question: What is the change in the velocity of the ball after 1 second? Type your answer here: m/s

d) Find the acceleration of the ball.

The ball is moving in a vertical direction so it is moving in accordance with gravitational acceleration. By definition gravitational acceleration is constant acceleration and will always be equal to 9.8 m/s2 regardless of what object is thrown or the initial velocity that object has.

CHECK YOURSELF! Think you know the answer? Enter it in the box below and press "Check!" to see if it's correct. Don't worry -- this is not a test, and if your answer is wrong, we'll tell you the solution!

 Question: What is the acceleration of the ball? Type your answer here: m/s2

DO A SIMILAR PROBLEM! Click here to get to a similar problem. This time, however, you will have to solve the problem all by yourself. Again, this is not a test and your submission will not be recorded.